1. Field of the Invention
The present invention relates to apparatus for playing back a rotary magnetic recording medium and, more particularly, to a playback apparatus for reproducing information out of a rotary magnetic recording medium, such as a magnetic disk, magnetic drum, while effecting tracking servo. Still more particularly, the present invention is concerned with a playback apparatus of the type which reproduces information out of concentric tracks provided on a magnetic disk while effecting tracking servo.
2. Description of the Prior Art
One of the achievements in the realm of modern imaging art is an electronic still camera system which uses a combination of an imaging device such as a solid state imaging device or an image pickup tube and a recording unit using a magnetic disk, which is a relatively inexpensive and large-capacity recording medium. In this system, an image is taken purely electronically in the form of electrical signals representative of a still picture to be recorded on a magnetic disk while the disk is rotated. The picture may be reproduced in the form of a soft copy using a television system or in the form of a hard copy using a printer by way of example.
A problem with recording media, particularly magnetic disks, applicable to magnetic recording of the kind stated is that their anisotropy, eccentricity, thermal expansion and other undesirable factors are apt to result in tracking error. During reproduction, tracking error often causes a track next to a desired track to be scanned simultaneously with the desired one and, thereby, brings about crosstalk between video signals stored in the nearby tracks.
One approach heretofore proposed to solve the above problem is a system which records a tracking signal together with information in a recording medium under application of tracking servo and, in the event of reproduction, effects tracking servo utilizing the tracking signal. However, it is impractical to install in a small-size light-weight recording apparatus such as a camera a tracking servo mechanism which requires accurate control.
In light of the above, a guard band system or a frequency modulation (FM) azimuth system may be used for a recording scheme. The guard band of FM azimuth recording scheme is successful in preventing a playback head from scanning a track adjacent to a desired one or, if scanned, from picking up a signal from the adjacent track, thereby compensating for some degrees of tracking error during reproduction.
Another known approach is a so-called envelope peak detecting autotracking control system. The principle of this system is such that during recording a record head is transported by a stepping motor at predetermined track pitches without effecting tracking servo and, during playback, an envelope of signals read out of each track is detected so as to identify an optimum track based on the peak of the envelope, thereby effecting tracking servo.
As described above, in an envelope peak detecting autotracking system, tracking control occurs on the basis of envelope levels of signals recorded in a recording medium. When a magnetic head is located at or near the positive peak of envelope levels, it is determined to be in an optimum tracking position. Whether or not the head has reached a peak or near-peak position is determined by comparing envelope levels at at least two nearby head positions so as to see whether or not they have no substantial difference.
A rotary magnetic recording medium is usually provided with a plurality of tracks at predetermined intervals. A rotary magnetic recording medium for use with an electronic still camera system, for example, may comprise a disk which is as small as about 50 millimeters in diameter and provided with fifty tracks at track pitches of about 100 microns, i.e. tracks each being 50-60 microns wide and guard bands each being 50-40 microns wide. In a playback unit, such a disk is rotated at a constant speed of, for example, 3,600 revolutions per minute so that video signals are reproduced at a field or frame rate.
For accurate tracking, it is advantageous to detect envelope levels at several positions, which the head encounters while transported in the radial direction of the disk, compare the detected envelope levels, and repeat such a procedure. A system preferable from the standpoint of practical use of the device is, for example, one which does not mute video signals and, instead, allows images to be reproduced on a video monitor while a playback head is transported from one track to another, or one which mutes video signals but cancels the muting mode during autotracking control, which occurs after a playback head has become on-track, so as to allow images to appear on a video monitor.
However, it has been found that performing ordinary color signal processing when a mute mode is cancelled at least upon the start of autotracking servo in order to produce images on a video monitor during the mute cancellation period gives rise to a problem as follows. Usually, a color signal processing circuit is inhibited from superposing a color burst on the back porch of a composite television signal in at least a part of horizontal blanking periods in a vertical blanking period in the raster scanning. Should such processing be performed even during autotracking servo, color signal processing would fail to build up to a sufficient degree and, thereby, disturb colors in reproduced pictures.